1. Field of the Invention
This invention relates to a voltage detecting apparatus, especially, the voltage detecting apparatus for detecting a voltage between both ends of a high-voltage battery for a car, which is structured by a plurality of unit cells connected in series to each other.
2. Description of the Related Art
A hybrid electric vehicle (HEV) driven to get her by an engine and an electric motor is becoming popular. The HEV includes two types of batteries, which are a low-voltage battery of about 12V for starting the engine and a high-voltage battery for driving the electric motor. The high-voltage battery supplies a high voltage by connecting in series unit batteries formed with secondary batteries, such as nickel-metal hydride batteries or lithium ion batteries, or unit cells formed with at least one secondary battery.
By repeating to charge and discharge the high-voltage battery, respective voltages of both ends of the unit cells change to have various values, that is each charged condition (State-Of-Charge: SOC) varies. For maintaining durability and the safety of each unit cell, charging the unit cell may be prohibited when the highest SOC (largest voltage value of the both ends) of the unit cells reaches a preset upper limit SOC (upper limit voltage value of the both ends), and discharging the unit cell may be prohibited when the lowest SOC (lowest voltage value of the both ends) of the unit cells reaches a preset lower limit SOC (lower limit voltage value of the both ends). In the HEV, by supplementing battery energy to a gasoline engine for hill-climbing, and by regenerating energy back to the battery when going downhill, that is by assisting/regenerating, the voltage value of the high-voltage battery is changed by driving conditions. Therefore, it is required that the voltage value of both ends of the high-voltage battery during the HEV driving is always monitored.
A voltage detecting apparatus for detecting a voltage value of both ends of each unit cell structuring the high-voltage battery is proposed in Patent Document 1. In the voltage detecting apparatus according to Patent Document 1, a main battery (high-voltage battery) is separated into a plurality of modules, and a circuit for detecting voltage value of both ends of each module and a circuit for detecting total voltage value of the whole main battery are separately provided. When the voltage value of both ends of each module and the total voltage value of the main battery are judged to be both abnormal values by judging respectively whether or not each voltage value is an abnormal value, malfunction of the battery is determined and charging/discharging of the main battery is stopped. Since the circuit for detecting voltage value of each module and the circuit for detecting total voltage value of the whole main battery are separately provided, when any one of the circuits malfunctions, the other circuit in a normal condition can detect the voltage value of the main battery to minimize the reduction of the performance thereof.
Accordingly the apparatus in Patent Document 1, by comparing the total voltage value detected by the circuit for detecting the total voltage value and a sum of each voltage value detected by the circuit for detecting a voltage value of each module, malfunction of the circuit for detecting total voltage value or the circuit for detecting voltage value of the module can be detected. Furthermore, by detecting each voltage value of each unit battery in the module, and comparing the voltage value of the module and a sum of each voltage value of each unit battery, malfunction of the circuit for detecting a voltage value of the module can be detected. Patent Document 1 is the Japan Published Patent Application No. H11-176480.